The present invention relates to a motor controller and an electric power steering apparatus.
In many conventional cases, a motor controller used in an electric power steering apparatus (EPS) includes detecting means that detects an anomaly such as a failed electric current flow, which is caused in any one of U, V, and W phases of a motor by a break of a power supply cable or contact failure of a driver circuit. Normally, if an anomaly is detected, the controller quickly stops operation of the motor and thus performs fail-safe.
If an EPS, such stopping of the motor greatly changes steering characteristics. Specifically, an increased steering force is required for the driver to accurately carry out steering. In this regard, for example, Japanese Laid-Open Patent Publication No. 2003-26020 discloses a motor controller that continuously operates a motor using two phases other than a phase in which the electric current flow has failed as electric current flowing phases, regardless of detection of the phase with such failure. In this manner, assist force is continuously applied to a steering system so as to suppress increase of load on the driver caused by the fail safe.
However, as in the conventional case, if the motor is continuously operated using the two phases other than the phase with a failed electric current flow as the electric current flowing phases after detection of the failure, a torque ripple may be caused and aggravate the steering comfort in the case illustrated in FIG. 9, in which a sinusoidal wave electric current is supplied to respective electric current flowing phases (in the example of FIG. 9, electric current flow in the U phase is failed while electric current flows in the V and W phases are normal).
When used in an EPS, which requires improved silence, a motor is usually driven by a sinusoidal wave electric current supply. Specifically, in many cases, detected phase electric current values of respective phases are converted into d-axis and q-axis electric current values of a d/q coordinate system. Feedback control is then performed in the d/q coordinate system in order to generate and output a motor control signal in accordance with which a sinusoidal wave electric current is supplied.
However, under the operation with two phases as described above, the actual q-axis electric current value changes in a sinusoidal wave manner even with the q-axis electric current command value, or a target control value of motor torque, maintained constant, with reference to FIG. 10. In other words, the motor is continuously operated without exerting its output performance to a maximally possible extent.